A heart rate sensor measures your heart rate. A heart rate sensor is either in a band around your chest or has been incorporated into your sports watch. A heart rate sensor in a chest strap connects with ANT + with your watch to read your heart rate. There are also heart rate monitors without a band that can be connected to your iPhone or Android smartphone via Bluetooth. Heart rate sensors in a chest strap often measure your heart rate more accurately than the heart rate sensor in a watch. It's good to use a heart rate sensor with chest strap next to your sports watch for extra accurate heart rate measurements.